USPS > Fedex/DHL/UPS in terms of pure safety.
Although you might have members SWEAR by Fedex and others, in my personal opinion, national USPS mail is the safest route. It's not fast but I've never had an item seized through this pathway. Generally because Fedex/UPS/DHL is responsible for collecting import tax and fees, so they will open any package that even looks expensive, i.e a watch.
Meanwhile USPS/US customs is flooded with mail and too many packages for them to check, they're not gonna bother with a small package with 1 watch, you may get unlucky, always a game of chance, but it's much lower than Fedex.